How Do Reverse Osmosis Filter Systems Work & What
Do They Do?
These are questions which we have
been asked numerous times when helping our customers select a filters
or a water filtration system which is right for them.
Below you will find diagrams and answers to those questions. If
you have other questions which are not answered, please visit our
FAQ page or Contact Us.
Producing Drinking Water Using Reverse Osmosis
Although Reverse Osmosis seems like a complex system
it is really a simple and straightforward water filtration process.
And it's not a new process. High-pressure (pump driven) reverse
osmosis systems have been used for years to desalinate*
water – to convert brackish or seawater to drinking water.
Having a better understanding of how a reverse osmosis system works
will eliminate the mystery and confusion you may feel when you look
at a reverse osmosis system -- with its many colored tubes and multitude
of filters. Read on to enhance your knowledge of residential reverse
The most important points to remember:
- All RO Systems work the same way.
- Most RO (Reverse Osmosis) systems look alike.
- All RO Systems have the same basic components.
- The real difference is the quality of the filters and membranes
inside the RO.
How the Reverse Osmosis System Works?
Reverse Osmosis is a process in which dissolved inorganic
solids (such as salts) are removed from a solution (such as water).
This is accomplished by household water pressure pushing the tap
water through a semi permeable membrane. The membrane (which is
about as thick as cellophane) allows only the water to pass through,
not the impurities or contaminates. These impurities and contaminates
are flushed down the drain.
For a definition of **Reverse Osmosis.
Ultimately, the factors that affect the performance
of a Reverse Osmosis System are:
- Incoming water pressure
- Water Temperature
- Type and number of total dissolved solids (TDS) in the tap water
- The quality of the filters and membranes used in the RO System
(see operating specs)
Diagram of a Reverse Osmosis Membrane:
What does a Reverse Osmosis System Remove?
A reverse osmosis membrane will remove impurities
and particles larger than .001 microns.
CHARACTERISTICS OF R.O. MEMBRANES
the Percent R.O. Membranes will remove
|85 - 94%
96 - 98%
94 - 98%
85 - 95%
94 – 98%
95 – 98%
95 – 98%
94 – 96%
96 – 98%
95 – 98%
92 – 96%
94 – 98%
96 – 98%
85 - 92%
94 – 98%
95 – 98%
95 – 98%
84 – 92%
85 – 92%
|% may vary based on membrane type
water pressure, temperature & TDS
Basic components common to all Reverse
- Cold Water Line Valve: Valve
that fits onto the cold water supply line. The valve has a tube
that attaches to the inlet side of the RO pre filter. This is
the water source for the RO system.
- Pre-Filter (s): Water from the
cold water supply line enters the Reverse Osmosis Pre Filter first. There
may be more than one pre-filter used in a Reverse Osmosis system.
The most commonly used pre-filters are sediment filters. These
are used to remove sand silt, dirt and other sediment. Additionally,
carbon filters may be used to remove chlorine, which can have
a negative effect on TFC (thin film composite) & TFM (thin
film material) membranes. Carbon pre filters are not used if
the RO system contains a CTA (cellulose tri-acetate) membrane.
- Reverse Osmosis Membrane: The
Reverse Osmosis Membrane is the heart of the system. The most
commonly used is a spiral wound of which there are two options:
the CTA (cellulose tri-acetate), which is chlorine tolerant,
and the TFC/TFM (thin film composite/material), which is not
- Post filter (s): After the water
leaves the RO storage tank, but before going to the RO faucet,
the product water goes through the post filter (s). The post
filter (s) is generally carbon (either in granular or carbon
block form). Any remaining tastes and odors are removed from
the product water by post filtration.
- Automatic Shut Off Valve (SOV): To
conserve water, the RO system has an automatic shutoff valve.
When the storage tank is full (this may vary based upon the
incoming water pressure) this valve stops any further water
from entering the membrane, thereby stopping water production.
By shutting off the flow this valve also stops water from flowing
to the drain. Once water is drawn from the RO drinking water
faucet, the pressure in the tank drops and the shut off valves
opens, allowing water to flow to the membrane and waste-water
(water containing contaminants) to flow down the drain.
- Check Valve: A check valve is
located in the outlet end of the RO membrane housing. The check
valve prevents the backward flow or product water from the RO
storage tank. A backward flow could rupture the RO membrane.
- Flow Restrictor: Water flow
through the RO membrane is regulated by a flow control. There
are many different styles of flow controls. This device maintains
the flow rate required to obtain the highest quality drinking
water (based on the gallon capacity of the membrane). It also
helps maintain pressure on the inlet side of the membrane. Without
the flow control very little drinking water would be produced
because all the incoming tap water would take the path of least
resistance and simply flow down the drain line. The flow control
is located in the RO drain line tubing.
- Storage Tank: The standard RO
storage tank holds up to 2.5 gallons of water. A bladder inside
the tank keeps water pressurized in the tank when it is full.
- Faucet: The RO unit uses its
own faucet, which is usually installed on the kitchen sink.
In areas where required by plumbing codes an air-gap faucet
is generally used.
- Drain line: This line runs from
the outlet end of the Reverse Osmosis membrane housing to the
drain. This line is used to dispose of the impurities and contaminants
found in the incoming water source (tap water). The flow control
is also installed in this line.
Diagram of a Reverse Osmosis
System with Basic Components:
Quality of RO Membranes and Filters – They're
not all alike!
While one RO System may look just like the next in
terms of design and components, the quality of those components
can be very different. These differences can have a significant
impact on the quality of the water the system produces.
Here are some examples of questions you might ask
and consequences associated with "less than desirable"
- Has the manufacturer used sound methods?
What types of welds have been used in these plastic products?
Will they allow contaminated water to bypass the filtration
system? Will they allow the system to leak?
- How has this filter or membrane been created?
Will it allow the water to 'channel' and, in effect, bypass
the removal component of this device?
- What about the quality of the 'fill'? Are
it's contents of a high enough quality to produce the expected
percentage of contaminant reduction? Carbon quality, for instance,
can have huge variances in reduction capability, reduction capacity,
and the sloughing of 'fines', which can prematurely clog or
foul the RO Membrane.
- What are the manufacturer's controls on tolerances
or variations in specifications? If this component
is rated as a 1-micron filter will it truly filter out everything
larger than 1 micron or will it only do the job 80% of the time?
And, what if it actually filters at a .5-micron rate? That will
stop the system from flowing -- clogging it and forcing filter
replacement? If this is a sediment filter and it fails the excess
sediment will clog or foul the RO Membrane.
- And in general - Are the materials used
in this product FDA or NSF (National Safety Foundation) approved?
If not, you might question their quality or performance ability.
So, it becomes clear that the quality
of the components is the key to an optimal functioning RO System.
Why and How To Increase the Gallon Per Day Capacity of A Reverse Osmosis Systems
The main reason to change to a higher flow reverse osmosis membrane is to improve the recovery rate which is to reduce the amount of time it takes to refill the storage tank. This insures that there is adequate water available during times of heavy usage or when the reverses osmosis system may feed more than one location such as an ice maker and a dispensing faucet.
Changing to a higher flow membrane has no effect on the quality of the water your reverse osmosis system makes or the length of time the reverse osmosis membrane will last.
The change to a higher capacity membrane is easy. You simply replace your old membrane with a new, higher capacity membrane, along with the correctly sized drain line flow restrictor. (Matching the membrane with the correctly sized drain line flow restrictor is important to ensure the proper product to waste ratio is meet. A mis-matched combination will allow either excess water to flow to the drain or cause premature fouling of the membrane.) Most standard reverse osmosis membrane housings will accommodate membranes ranging in capacities from 10 – 150 gallons per day.
Flow Control Installation Instructions
A complete listing of standard residential reverses osmosis membranes and corresponding drain line flow controls is on our membrane page or review the flow control guideline chart for residential reverse osmosis membranes on our parts page.
Where Can I purchase a Quality Reverse Osmosis System
or replacement filters and membranes?
We carry replacement filters and membranes for most
major brands of Reverse Osmosis systems. View our list of Reverse
Osmosis Filters by brands that we carry.
We also have new Reverse Osmosis Systems available
to purchase. See the features and benefits of each type of Reverse
*Desalinate - The removal of dissolved inorganic
solids (salts) from a solution such as water to produce a liquid
which is free from dissolved salts.
**Osmosis – The natural
tendency for water molecules to pass through a semi permeable membrane,
from the side low in dissolved impurities to the side high in dissolved
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